1. Field of the Invention
The invention relates to communications systems using CDMA (Code Division Multiple Access). More particularly it relates to the use of open-loop timing control for the reverse link of a synchronous CDMA system.
2. Background Art
Synchronous CDMA is an efficient multiple access scheme wherein users are allocated PN codes which are optimized to have a minimal cross correlation when time-aligned with each other. In order to benefit from these minimal cross correlations, these PN codes must be time aligned at the receiver. In the forward channel (radio base unit, RBU, to subscriber unit, SU, direction) it is easy to time align the codes because they are all modulated at the RBU. In the reverse direction, this is more difficult because the SUs are distributed throughout the cell and their transmissions must be coordinated so that their signals all arrive synchronously at the RBU. To accomplish this reverse channel synchronization, an initial coarse synchronization is performed (See U.S. Pat. No. 5,825,835). Next, a fine synchronization is performed using a timing control loop (see U.S. Pat. No. 5,867,525) which is a closed loop.
The closed-loop timing control loop operates using feedback from the RBU to the SU. The time-of-arrival (TOA) of the reverse channel signal from an SU is estimated and compared to the RBU""s desired TOA. The timing error is fed back to the SU via the forward channel and the SU responds by adjusting its transmitter""s timing in an attempt to drive the TOA error at the RBU to zero. Closed-loop timing control is adequate for a non-mobile synchronous CDMA system such as the Prime Wave 2000 system. However, each iteration of the closed-loop timing control loop takes a significant amount of time. As a result, if it is desirable to make the system capable of supporting mobile SUs, then the closed-loop timing control loop may be too slow to keep the users adequately synchronized.
In a mobile system, the range, and thus the propagation time between the RBU and SU will vary. As a result, the SU will need to adjust its transmitter clock more frequently than the closed loop is able to iterate. The Prime Wave 2000 system offers timing control updates about once every quarter of a second. Since the chip rate of Prime Wave 2000 is 2.72 Megachips/sec., the wavelength of a chip is 110 meters. This implies that a vehicle moving at a velocity of greater than 79 km/hour with respect to the RBU would see a 1/20 chip range change every quarter of a second. This would result in a change in TOA at the RBU of 1/10 chip every quarter of a second. This is probably the extreme limit of what the closed-loop timing control loop can track, and even this relative velocity is probably too great. If the SUs move at a higher rate, one possible approach is to speed up the closed-loop timing control loop. However, circuitry for achieving this result may not be available, or may be prohibitively expensive, especially for use in SUs, where cost can be a major issue.
It is an object of the invention to use information from the forward channel TOA (time of arrival) to determine the appropriate reverse channel transmitter adjustments.
It is another object of the invention to use a GPS receiver to provide a stable clock reference against which the received chip clock from the forward channel may be compared.
It is yet another object of the invention to provide a reference frame in which to measure forward channel TOA by using a GPS receiver to provide clocks to the RBU and SU.
In accordance with the invention, generally, an open-loop timing control loop may be used to augment the closed-loop approach. Thus, open-loop timing control may use information available at the SU to make corrections to the SU""s transmitter clock. Since open-loop updates do not require feedback from the RBU, they can typically be made much more often than a closed-loop update. As a result, if open-loop corrections are used along with the standard closed-loop corrections, then an SU will be capable of remaining synchronous in a much more dynamic environment.
Generally, according to the invention, there is provided a method for maintaining synchronization in a CDMA communication system comprising operating closed loop timing control between a radio base unit and the subscriber unit, operating open loop timing control between the radio base unit and the subscriber units; and using timing information derived from the open loop to correct the closed loop. The open loop is faster in response than the closed loop. Information from a short time constant control loop and information from a long time constant control loop is combined to derive the open loop timing information. The open loop timing information is combined with a closed loop timing control update to provide information to correct the closed loop. The information used to correct the closed loop is provided as a time offset signal to adjust phase of a transmitter in a subscriber unit.
In accordance with the invention the open loop timing control may be generated by using GPS data. The data may include subscriber unit position data from a GPS receiver in the subscriber unit. The data may further include subscriber unit velocity data. Projected position data of the subscriber unit may be calculated based on the position data and the velocity data. The projected position data and the closed loop timing control update data may be used to derive a time offset signal to adjust phase of a transmitter in the subscriber unit.
In accordance with the invention, the radio base unit may receive GPS data. This data may be received by the radio base unit from a GPS receiver located in the radio base unit. The GPS data received by the radio base unit is synchronous with GPS data received by the subscriber unit. If the radio base unit is fixed in position, the GPS data may be stored in a memory in the radio base unit. If the radio base station is moved to a new location, new GPS data is loaded into the memory.
The invention is also directed to a radio base station for use in a CDMA communications system, comprising a receiver for receiving signals from subscriber units; a transmitter for transmitting signals to the subscriber units; and a source of position data associated with the radio base station for providing data concerning the position of the radio base station, the position data being transmitted by the transmitter to the subscriber units. The source of position data may be a GPS receiver or a position data memory.
The invention is further directed to a subscriber unit for a CDMA communications system, comprising a receiver for receiving signals from a radio base station; a transmitter for transmitting signals to the radio base station; a closed loop timing control circuit for adjusting timing of signals transmitted by the transmitter; an open loop timing control circuit for adjusting timing of signals transmitted by the transmitter; and a circuit for correcting the closed loop timing control based on information from the open loop timing control. The open loop timing control is faster in response than the closed loop timing control.
The subscriber unit further comprising a short time constant control loop; a long time constant control loop; and a first means for combining information from the short time constant loop and from the long time constant loop to derive the open loop timing information.
The subscriber unit may further comprise means for supplying a timing control update for the closed loop; and a second means for combining the timing control update with the open loop timing information to provide information to correct the closed loop. The second means may be a microprocessor. The second means may provide a time offset signal to correct the closed loop.
The subscriber unit may further comprise a source of GPS data; the GPS data being used to generate the open loop timing control. The source of GPS data may be a GPS receiver. It may provide GPS position data of the subscriber unit. The data may further include subscriber unit velocity data.
The subscriber unit may further comprise means for calculating projected position data of the subscriber unit based on the position data and the velocity data; and phase adjusting means for using the projected position data and closed loop timing control update data to derive a time offset signal to adjust phase of the transmitter in the subscriber unit.